1. Field of the Invention
The present invention relates to illumination devices and liquid crystal display devices, and more particularly, it relates to an illumination device which uniformly and brightly illuminates a large area with a single light source, and also to a liquid crystal display device using the same.
2. Description of the Related Art
A conventional front light of a liquid crystal display device has a unit structure including a light source, an intermediate light guide, a light guiding plate, and a casing integrally holding these components and having a reflective inner surface.
FIG. 21A is a perspective view illustrating the structure of such a liquid crystal display device and FIG. 21B is a plan view illustrating the structure of the liquid crystal display device shown in FIG. 21A. The liquid crystal display device shown in these figures is formed of a liquid crystal display unit 120 and a front light 110 disposed close to the front surface of the liquid crystal display unit 120. Although not illustrated in detail in the figures, the liquid crystal display unit 120 is of a reflective type performing a display by reflecting light incident on its front surface and is formed of mutually opposing upper and lower substrates 121 and 122, and a liquid crystal layer held therebetween. That is, the display is performed by controlling the alignment of the liquid crystal layer so as to change the transmitting state of the incident light.
The front light 110 is formed of a flat light guiding plate 112 including a side surface 112a, a rod-shaped intermediate light guide 113 disposed on the side surface 112a, and a light emitting element 115 disposed on one end surface of the intermediate light guide 113. The upper surface of the light guiding plate 112 is formed so as to have a plurality of mutually parallel, prismatic projections 114, each having a wedge shape in plan view. These projections 114 are formed at a slightly oblique angle with respect to the side surface 112a of the light guiding plate 112 so as to prevent moire fringes.
In the front light 110, light emitted from the light emitting element 115 is incident on the side surface 112a of the light guiding plate 112 via the intermediate light guide 113 and is introduced into the light guiding plate 112. Then, the light is reflected at the inner side of the prismatic upper surface of the light guiding plate 112 so as to change its propagating direction and is incident on the liquid crystal display unit 120 through the lower surface (shown in FIG. 21A) of the light guiding plate 112.
In portable electronic apparatuses such as a portable information terminal and a portable game machine, the usability of these apparatuses is largely affected by a battery use time. Therefore, in order to reduce electric power consumption of a front light of a liquid crystal display device used as a display portion of each portable electronic apparatus, a single-light-source front light such as the front light 110 shown in FIG. 21A and having the light emitting element 115 as a single light source has been used. That is, it is required to reduce electric power consumption by reducing the number of light emitting elements. Also, it is required to make the front light 110 thinner to the order of 1 mm due to the requirement for compactness of the portable electronic apparatuses.
However, such a single-light-source front light including a combination of a thin light guiding plate and a single light emitting element has been almost impossible to uniformly and brightly illuminate an area of a several-inch or larger display screen. More particularly, when the front light 110 has the light emitting element 115 disposed on its one side as shown in FIG. 21A, the intermediate light guide 113 is required to make incident light uniform along the side surface 112a of the light guide 112 so as to uniformly introduce the incident light emitted from the light emitting element 115 into the light guiding plate 112. However, since it is difficult for the intermediate light guide 115 to make light uniformly incident on the light guiding plate 112, it is very difficult for the light guiding plate 112 to uniformly emit light from its entire surface. As a result, the front light 110 has a problem in which, in an especially remarkable case, a triangular dark area 118 as shown in FIG. 21B occurs in the vicinity of a side (the left side in the figure), close to the light emitting element 115, of the light guiding plate 112, thereby causing a reduced visibility of a liquid crystal display device.
Also, when the light guiding plate 112 is made thin so as to make a portable electronic apparatus thin and compact, light propagating in the light guiding plate 112 is likely to leak out therefrom upon reflection against the inner surface thereof, thereby causing a problem in which the farther an area of the light guiding plate 112 is away from the light emitting element 115, the drastically less the quantity of light is incident on the area.
Although a front light using a single light emitting element as a light source has been increasingly required as described above, a thin front light able to uniformly and brightly illuminate a large area has not been achieved.